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In the process of decryption a data, I am getting this two definitions confused. I would really appreciate a better understanding to these two different description.
Already searched google, to see if i can get more understanding, but still confused with the ones i got.
Thanks so much for your time.
WRT DUKPT key varients:
Here is a table that shows the DUKPT variants. You really should get ANSI X9.24, Part 1 document and read the DUKPT section.
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I've been reading up on file encryption lately, and In many places I've seen warnings that encrypted files are susceptible to decryption by people so inclined regardless of encryption algorithm strength.
However, I can't get my head around how someone would go about attempting to decrypt an encrypted file.
For example, lets say you've got an encrypted file and you'd like to know it's contents. You have no idea what the key used to encrypt the file is, nor the encryption algorithm used. What do you do? (Assume for this example that the encryption algorithm is a symmetric-key algorithm such as AES-256, I.E. a file encrypted with key which requires said key to decrypt it).
Additionally, how would your approach change if you knew the encryption algorithm used? (Assume in this case that the encryption algorithm used is AES-256, with a random key + salt).
There's two ways to answer this question, in the literal sense of how a perfect crypto system is attacked, and how real world systems are attacked. One of the biggest problems you'll find as you begin to learn more about cryptography is that selecting algorithms is the easy part. It's how you manage those keys that becomes impossibly difficult.
The way in which you attack the basic primitives depends on the type of algorithm. In the case of data encrypted by symmetric ciphers like AES you use Brute force attacks. That is, you effectively try every key possible, until you find the right one. Unfortunately, barring changes in the laws of physics trying every possible 256-bit key can't be done. From Wikipedia: "A device that could check a billion billion (10^18) AES keys per second would in theory require about 3×10^51 years to exhaust the 256-bit key space"
The problem with your question about coming across a seemingly encrypted file, with no knowledge of the methods used, is that it's a bit of a hard problem known as a Distinguishing Attack. One of the requirements of all modern algorithms is that their output should be indistinguishable from random data. If I encrypt something under both AES and Twofish, and then give you some random data, absent any other information like headers, there's no way for you to tell them apart. That being said....
You asked how knowledge of the algorithm changes the approach. One assumption cryptographers usually make is that knowledge of the algorithm shouldn't affect security at all, it should all depend on the secret key. Usually whatever protocol you're working with will tell you the algorithm specifications. If this wasn't public, interoprobility would be a nightmare. Cipher Suites, for example, are sets of algorithms that protocols like SSL support. NIST FIPS and the NSA Suite B are algorithms that have been standardized by the Federal Government, that most everyone follows.
In practice though, most crypto-systems have much larger problems.
Bad random number generation: Cryptography requires very good, unpredictable random number generators. Bad random number generators can completely collapse security, as in the case of Netscape's SSL implementation. You also have examples like the Debian RNG bug, where a developer changed code to satisfy a memory leak warning, which ultimately led to Debian generating the same certificate keys for every system.
Timing Attacks: Certain operations take longer to execute on a computer than others. Sometimes, attackers can observe this latency and deduce secret values. This has been demonstrated by remotely recovering a server's private key over a local network.
Attacks against the host: One way to attack a cryptosystem is to attack the host. By cooling memory, its contents can be preserved and inspected in a machine you control.
Rubber hose cryptanalysis: Maybe one of the easiest attacks, you threaten the party with physical harm or incarceration unless they reveal the key. There has been a lot of interesting case law on whether or not courts can force you to reveal crypto keys.
AES256 is effectively unbreakable.
From http://www.wilderssecurity.com/showthread.php?t=212324:
I don't think there's any credible speculation that any agency can
break a properly implemented AES. There are no known cryptanalytic
attacks, and actually bruteforcing AES-256 is probably beyond human
capabilities within any of our lifetimes. Let's assume that 56 bit DES
can be bruteforced in 1 sec, which is a ridiculous assumption to begin
with. Then AES-256 would take 2^200 seconds, which is 5 x 10^52 years.
So, you can see that without any known weakness in AES, it would be a
total impossibility within any of our lifetimes, even with quantum
computing. Our sun will explode, approximately 5 billion years from
now, before we obtain enough computing power to bruteforce AES-256
without a known weakness. IF a weakness in AES is never found, there
is absolutely no reason to ever look for another cipher besides AES.
It will suffice for as long as humans occupy the planet.
With basic Brute force attack for example. You ask a software to try every single combination between 1 character to 15 character with a-z A-Z 0-9 and wait.
The software will start with 0 to 10... then 0a, 0b, 0c until it finds the password. Wikipedia will give you more detail.
I partially agree with Andrew and partially with Jeremy.
In the case, if encryption key is generated correctly (random generated or based on complex password, good key derivation function and random salt) then AES256 is effectively unbreakable (as Andrew said)
On other hand, if a key isn't correctly generated. As example, just straight hash of 4 digit's PIN password, brute force could be very efficient.
Regarding "You have no idea what the key used to encrypt the file is, nor the encryption algorithm used. "
In most case, encrypted files has a header or a footer which specify something (an application used to encrypt a file, encryption algorithm or something else).
You can try to figure out algorithm by padding (as example 3DES has padding and AES has different padding)
I am reading about cryptography I was thinking about these properties of AES (that I use):
same message = same ouput
no message length secrecy
possible insecurity if you know the messages (does this actually apply to AES?)
I hear that AES is secure, but what if I want to theoritcaly improve these properties?
I was thinking I could do this:
apply encryption algorithm A
XOR with random data D (making sure the output looks random in case of any cipher)
generate random data that are longer than the original message
use hashing function F to allocate slots in random data (this scrambles the order bytes)
Inputs: Encryption algorith A, Data to XOR with D and a hashing function F
My questions are
does the proposed solution theoreticaly help with my concerns?
is this approach used somewhere?
Possible enhancements to this approach
I could also say that the next position chosen by hashing function will be altered using a checksum of the last decoded byte after the XOR step (that way the message has to be decoded from beginning to end)
If I was to use this to have conversation with someone, the data to XOR with could be the last message from the other person, but thats probably a vulnerability.
I am looking forward to your thoughts!
(This is only theoretical, I am not in need of more secure encryption, just trying to learn from you guys.)
Yeah.
Look. If you want to learn about cryptography, I suggest you read Applied Cryptography. Really, just do it. You will get some nice definitive learnings, and get an understanding of what is appropriate and what is not. It specifically talks about implementation, which is what you are after.
Some rules of thumb:
Don't make up your own scheme. This is almost universally true. There may be exceptions, but it's fair to say that you should only invent your own scheme if you've thoroughly reviewed all existing schemes and have specific quantifiable reasons for them not being good enough.
Model your attacker. Find out what scenarios you are intending to protect against, and structure your system so that it works to mitigate the potential attacks.
Complexity is your enemy. Don't make your system more complex then it needs to be.
Stay up to date. You can find a few mailing lists related to cryptography and (and hashing) join them. From there you will learn interesting implementation details, and be aware of the latest attacks.
As for specifically addressing your question, well, it's confusing. I don't understand your goal, nor do I understand steps 3 and 4. You might like to take a quick look here to gain an understanding of the different ways you can use a given encryption algorithm.
Hope this helps.
You assumptions are incorrect.
same message != same output
The output will not be the same if you encrypt the same message twice.
This is because you are suppsed to use different IVs'.
Message length can be hidden by adding random data to the plaintext.
Attacks have been demonstrated against AES with lesser number of rounds.
Full-round AES has not been compromised in any way.
Other than that I suggest you follow Noon Silks recommendation and read Applied Cryptography.
What's the point of the random data XOR? If it's truly random, how will you ever decrypt it? If you're saying the random data is part of the key, you might as well drop AES and use only the truly random key - as long as it's the same length (or longer than) the data and is never used more than once to encrypt. It's called a one-time pad, the only theoretically unbreakable encryption algorithm I know about.
If the random bits are pseudo-randomly generated, it's highly unlikely that your efforts will yield added security. Consider how many talented mathematicians were involved in designing AES...
EDIT: And I too highly recommend Applied Cryptography, it's an actually very readable and interesting book, not as dry as it may sound.
I realize this question might not be that programming related, and that it by many will sound like a silly question due to the intuitive logical fault of this idéa.
My question is: is it provable impossible to construct a cryptographic scheme (implementable with a turing-complete programming language) where the encrypted data can be decrypted, without exposing a decryption key to the decrypting party?
Of course, I can see the intuitive logical fault to such a scheme, but as so often with formal logic and math, a formal proof have to be constructed before assuming such a statement. Is such a proof present, or can it easely be constructed?
Thank you for advice on this one!
Edit: Thank you all for valuable input to this discussion!
YES!!! This already exists and are called zero knowledge protocols and zero knowledge proofs.
See http://en.wikipedia.org/wiki/Zero-knowledge_proof
However, you have to have a quite a good background in mathematics and crypto to understand the way it works and why it works.
One example of a zero knowledge protocol is Schnorr's ZK protocol
No; but I'm not sure you're asking what you want to be asking.
Obviously any person who is decrypting something (i.e. using a decryption key) must, obviously, have the key, otherwise they aren't decrypting it.
Are you asking about RSA, which has different keys for decrypting and encrypting? Or are you asking about a system where you may get a different (valid) result, based on the key you use?
If by "decrypted" you just mean arrive at the clear text in some way, then it is certainly possible to create such a cryptographic scheme. In fact it already exists:
Take an asymmetric encryption scheme, eg: RSA where you have the public key but not the private key. Now we get a message that's been encrypted with the public key (and therefore needs the private key to decrypt it). We can get the original message by "brute force" (yes, this'll take an enormously long time given a reasonable key/block size) going through all possible candidates and encrypting them ourselves until we get the same encrypted text. Once we get the same encrypted text we know what the decrypted text would be without ever having discovered the private key.
Yes.
Proof: Encryption can be considered as a black box, so you get an input and an output and you have no idea how the black box transforms the input to get the output.
To reverse engineer the black box, you "simply" need to enumerate all possible Turing machines until one of them does produce the same result as the one you seek.
The same applies when you want to reverse the encryption.
Granted, this will take much more time than the universe will probably live, but it's not impossible that the algorithm will find a match before time runs out.
In practice, the question is how to efficiently find the key that will decode the output. This is a much smaller problem (since you already know the algorithm).
It's called encoding.
But everyone with the encoding algorithm can "decrypt" the message. This is the only way of keyless encryption.
Imagine I have this:
$cdata = AES_256($data, $pass);
AES_256 implements the AES algorithm.
If I know the content of $cdata and the content of $data and also have
the AES_256() code, can I reverse engineer and find $pass?
Simple answer: NO.
This has been tested, and mentioned in the Wiki link.
A related-key attack can break up to 9
rounds of 256-bit AES. A
chosen-plaintext attack can break 8
rounds of 192- and 256-bit AES, and 7
rounds of 128-bit AES, although the
workload is impractical at 2128 -
2119.
Or put it another way: you have a better chance of being struck by lighting... on the same day you win the Lottery, than breaking it!
This is called a known-plaintext attack. A good cipher like AES should be immune to it, as the others explained.
If $pass is actually a password and not a 256-bit key, you may be in luck.
While it is far from trivial to perform, a brute-force attack against a normal password is much faster than brute-forcing a 256-bit key.
So modify one of the many password-brute-forcing tools, and you have a attack that (depending on the strength of the password) might take weeks to several years - but that is fast compared to 3x10^51 years...
Another quote, from Wikipedia:
AES permits the use of 256-bit keys.
Breaking a symmetric 256-bit key by
brute force requires 2^128 times more
computational power than a 128-bit
key. A device that could check a
billion billion (10^18) AES keys per
second would require about 3 x
10^51 years to exhaust the 256-bit
key space.
Brute forcing when you know the original text might be faster but still, 3 x 10^51 years is a long time. Plus there's the problem of probably not having a device that can check a billion billion (10^18) keys/second.
In short: everything is possible, but this is not feasible in the world we are now living in.
You could brute force it, but it would take a long time. As in decades or even longer. That's the point of encryption algorithms like AES.
AES, like all good crypto algorithms, doesn't rely on security through obscurity.
In other words, there are no "secrets" in the code, so you having the code won't help you particularly.
Known plaintext is a separate issue, which I don't know much about so I'll leave that up to the other answerers.
Of course not - the only approach is brute force.
Do you really think NIST is so stupid as to choose a cipher that is so easily cracked for a new standard?
with the power of super computers the time to crash AES encryption with be dramatically shortened.... I heard...
2x2^256 possible combinations is a lot to bruteforce. But bruteforcing is the only way. It would actually take about 3 decades. AES is the best Encryption possible right now I'd say. But that would only take that much time using a CPU. Because GPU's (Graphic Processing Units) are strictly math based, people have been making programs that only use the GPU to crack math based algorithms much more quickly than a CPU could. In other words AES might not last 3 decades. If only eternity codes were possible. Well looks like dynamic encryption may be the only way people can really hide their information in the near future.